The present invention relates, in general, to the field of distance or range measuring equipment. More particularly, the present invention relates to a laser based range finder which may be inexpensively produced yet provides highly accurate precision range measurements of up to 1000 yards or more with a resolution of less than 1 yard. The laser range finder herein disclosed has a number of user selectable target acquisition and enhanced precision measurement modes which may viewed on an in-sight display during aiming and operation of the instrument. Extremely efficient self-calibrating precision timing and automatic noise threshold circuits incorporated in the design provide a compact, low-cost, highly accurate and reliable ranging instrument for a multitude of uses.
Laser based distance and range measuring equipment have been used for a number of years to provide extremely accurate distance measurements to a remote target or object. A representative instrument is the Criterion(trademark) 100 laser range finder developed and marketed by Laser Technology, Inc., assignee of the present invention. Although a highly accurate and reliable device, its great distance ranging capability and inherent complexity translates to a cost and form factor most suitable only for certain specific applications. A need therefore exists for a laser based range finder of perhaps more limited range, which can be economically manufactured as a rugged, compact unit to provide accurate distance measurement capabilities in other less stringent types of applications.
Herein disclosed is a precise, yet accurate and reliable laser range finder which may be economically produced and is adapted to individual portable use in a unit potentially weighing less than a pound with an on-board battery based power supply. Moreover, the compact instrument herein provided has a number of user selectable target acquisition operational modes which may be invoked depending on the distance, type and reflectivity of the target being sighted.
Through the use of an in-sight display, distance or range information can be shown while the user may also view and select the instrument""s mode of operation through successive actuations or a push button mode switch while simultaneously sighting the target object. A precision mode of operation may also be invoked in which an even more precise measurement to an object may be achieved following an initial measurement together with the visual indication of xe2x80x9cprecision flagxe2x80x9d on the in-sight display.
A highly precise range measurement is made possible through the use of a novel and efficient timing circuit which makes use of the instrument""s internal central processing unit crystal oscillator. A likewise unique automatic noise threshold determining circuit allows for instrument operation with a low signal-to-noise ratio to optimize sensitivity and performance in conjunction with a processor based pulse discrimination procedure which, nevertheless assures accurate range measurements.
The unit herein disclosed can be utilized in a multitude of endeavors including such recreational activities as golf where it can be utilized to very accurately determine the distance to a flag or pin as well as to trees and other natural objects. The principles of the invention are further applicable to the design of a laser based xe2x80x9ctape measurexe2x80x9d where ranges can be precisely measured with resolutions of on the order of an inch or less.
Specifically disclosed herein is a self-calibrating, precision timing circuit and method for determining a range to a target based upon a flight time of a pulse toward the target. The circuit comprises means for initially establishing first and second reference voltage levels together with means for unclamping the second reference voltage level and means for allowing the second reference voltage level to then diminish at a first rate to the first reference voltage level. Further provided are means for storing a first reference time extending from the step of unclamping until the first and second reference voltage levels are determined to be equal. Means are also provided for then re-establishing the first and second reference voltage levels together with means for again unclamping the second reference voltage level. Additional means are provided for increasing the second reference voltage level at a second higher rate than the first rate for a predetermined period of time to establish a third reference voltage level together with means for then allowing the third reference voltage level to diminish at the first rate to the first reference voltage level at which time, a second reference time extending from the step of again unclamping until the first and third reference voltage levels are equal is additionally stored. The first and second reference voltage levels are again re-established and the second reference voltage level is further unclamped. Means are provided for again increasing the second reference voltage level at the second higher rate for a period of time related to the flight time of the pulse to the target to establish a fourth reference voltage level, together with means for then allowing the fourth reference voltage level to diminish at the first rate to the first reference voltage level. A third reference time extending from the unclamping of the second reference voltage level until the first and fourth reference voltage levels are equal is then stored and the range to the target may be computed as proportional to the quantity of the (third reference time minus the first reference time) divided by the quantity of the (second reference time minus the first reference time).
In a particular embodiment the establishing means may comprise a transistor switch for coupling a capacitor to a source of the second voltage while the unclamping means may comprise a second transistor switch for decoupling the capacitor from the second voltage source. The allowing means may comprise a third transistor switch coupling a resistor to the capacitor to bleed off the charge therefrom.
The means for increasing the second reference voltage level may comprise means for applying a charge to the capacitor at the second rate and the predetermined time period specified may be determined by reference to a crystal oscillator. In a particular embodiment, the second charging rate may be substantially 1000 times the first discharging rate.